The invention relates to methods and apparatus for making electrical connections in remote locations, e.g., inside an oil well. More specifically, the invention relates to a mechanical latch for connecting two mating parts of an electrical connector.
When a well is drilled, it is common to survey, or xe2x80x9clog,xe2x80x9d certain sections of the well. Well logging involves obtaining and recording data related to one or more characteristics of the formations penetrated by the well. Many types of logs, e.g. mechanical, acoustic, electric, and radioactivity, can be made using appropriate logging tools. In wireline logging, the logging tools are deployed into the well by means of an armored electrical cable, or wireline, wound on the drum of a winch. The logging tools make measurements at selected depths of the well and send signals to a logging unit at the earth""s surface through the cable. With the aid of gravity, the logging tools can be lowered into vertical wells by simply unwinding the cable from the winch drum. In horizontal or highly-deviated wells, however, gravity is frequently insufficient to move the logging tools to the depths to be logged. In these situations, it is sometimes necessary to push the logging tools along the well with drill pipe, coiled tubing or the like.
Wireline logging with drill pipe is complicated by the presence of the cable. It is cumbersome and dangerous to string the cable through all of the drill pipe before lowering the logging tool into the well. Some deployment systems have therefore been developed, such as one offered under the trade name Tough Logging Conditions System (TLCS) by Schlumberger Technology Corporation, Houston, Tex., which allow an electrical connection to be made between the logging tool and the cable after the logging tool has been lowered to the desired depth in the well. In these deployment systems, the logging tool is deployed with standard drill pipe. Then, the cable is run through the drill pipe and connected to the logging tool. After logging, the cable is detached from the logging tool and removed before the logging tool is retrieved.
In the TLCS and other deployment systems, the cable is remotely connected to the logging tool using a downhole connector. One mating part of the connector is mounted inside a docking head, which is attached to the logging tool. The logging tool is then lowered into the well on drill pipe. The other mating part of the connector is mounted in a pump-down head, which is attached to the end of the cable. The pump-down head is forced down the drill pipe with a flow of fluid, such as drilling mud, that circulates out of holes at the bottom of the drill pipe and into the well. An electrical connection is established when the mating parts come in contact. This connection is typically referred to as a xe2x80x9cwet connectionxe2x80x9d because it is made in the flow of fluid, which is often conductive and challenges the reliability of the electrical connection. A mechanical latch joins the mating parts together and maintains the integrity of the joint during the logging operation. In most systems, the latching is established using the kinetic energy of the pump-down head that is pushed down by the circulating fluid. At the completion of the logging operation, the pump-down connector head is unlatched from the docking head, allowing the cable to be pulled out of the drill pipe. The pump-down head is usually unlatched from the docking head by pulling on the cable with a predetermined amount of tension.
In deeper wells, it becomes more difficult to meet all functional requirements for the mechanical latch. In particular, existing wet connector latch mechanisms do not meet two conflicting requirements for the strength of the mechanical latch. On one hand, the tensile force available for unlatching at the pump-down head diminishes quickly with depth and well curvature due to friction between the cable and the drill pipe and the weight of the cable itself. Thus, a weak latch that can be easily released is required under these conditions. On the other hand, pushing the pipe down into a deep and/or highly-deviated well is often characterized by stick and slip motion of the drill pipe. This uneven motion creates very short-lived, but high-amplitude, forces on the mechanical latch because the pipe motion and cable motion do not correspond. In order to avoid accidental unlatching due to these forces, the latch has to be strong.
Mechanical latches for downhole wet connectors have been disclosed and are commercially available. U.S. Pat. No. 5,967,816 issued to Sampa et al. discloses a latch mechanism which includes a three-finger latch collet and a latch ring. The latch collet is attached to the docking head, while the latch ring is part of the pump-down head. During latching, the latch ring forces the fingers open and passes through the collet. The fingers then close behind the ring, preventing the pump-down head from separating from the docking head. The unlatching is conducted by applying tension to the logging cable. When this force is strong enough to exceed the yield strength of the ring material at the points of contact, the fingers break loose, destroying a portion of the ring. The mechanism has the advantages of simplicity and reliability and has been very successful commercially. In addition, its release forces are highly predictable. Different levels of force can be achieved by varying the strength of the latch ring. However, the number of latch/unlatch cycles is limited because the ring suffers substantial damage every time it is unlatched. Further, the unlatching force is constant and cannot be adjusted once the latch is downhole. Hence this mechanism reaches the limit of its usefulness at a certain well depth.
U.S. Pat. No. 4,799,546 issued to Hensley et al. and U.S. Pat. No. 4,700,778 issued to Smith et al. disclose latch mechanisms based on J-shaped slots, or J-slots. These latch mechanisms typically include protrusions on one mating part of the connector and J-slots cut in the other mating part of the connector. One mating part of the connector is attached to the pump-down head, while the other mating part is mounted in the docking head. During latching, the protrusions engage the J-slots and then slide along them, forcing the pump-down head to rotate a predetermined amount. At the end of the travel, a spring pushes back the mating part containing the protrusions. The shape of the J-slots prevent the protrusions from traveling back along the same path. Instead, the protrusions are forced towards a different section of the J-slot, thus locking the two mating parts of the connector together. The protrusions can be separated from the J-slots by either tensioning and slackening of the cable or by pushing the pump-down head down. This forces the protrusions to travel along a third section of the J-slot, which frees the protrusions from the J-slots and allows the pump-down head to be separated from the docking head. The advantage of the J-slot system is that it allows multiple latch/unlatch cycles. Another advantage is that no elements of the system are destroyed, and there is no risk of leaving debris in the well. The system is relatively complex, however, and there is a higher risk of accidentally unlatching due to an unintended pull on the cable caused, for example, by stick-slip motion of the drill pipe.
U.S. Pat. No. 5,058,683 issued to Godfrey et al. discloses a J-slot latch mechanism that has reduced risk of accidental unlatching. In this mechanism, the shape of the J-slot is modified such that multiple tensioning and slackening cycles on the cable are required before the latch is released. This mechanism, however, does not completely eliminate the occurrence of an accidental unlatch. The Godfrey et al. patent also discloses an electrically activated wet connector latch which uses electromagnetic devices to create the force holding the two mating parts of the wet connector together. This connector has many potential advantages, the most important of which is that the latch can be controlled remotely. However, this latch mechanism has not received wide commercial acceptance due to its complexity, difficulties with downhole power supplies, and safety concerns, especially when the tool string contains perforating guns.
U.S. Pat. No. 5,158,142 issued to Miszewski et al. discloses an apparatus for releasing a pipe string from an object stuck downhole. This apparatus uses a hydraulic time-delay mechanism that selectively releases only when a predetermined tension is applied to the pipe string for a predetermined amount of time. This mechanism does not release under accidental high tensile loads, such as those that may result from shooting perforating guns. However, the mechanism releases when a much smaller tensile force is applied for much longer periods of time. This mechanism is not suitable for wet connectors because it cannot latch, it only releases. Hence, it can only be used once, and the mechanical connection has to be established in some other way.
Therefore, what is needed is a latch mechanism for electrical connectors which can be easily released, does not accidentally unlatch, and has multiple latch/unlatch cycles.
In one aspect, the invention relates to a connector latch which comprises a housing body having an internal cavity, a grippable member slidably coupled to the housing body, and a hydraulic mechanism on the housing body for delaying sliding of the housing body relative to the grippable member when an external force is applied to the housing body.
In another aspect, the invention relates to a connector latch which comprises a housing body having an internal cavity, a grippable member slidably coupled to the housing body, a gripping member for selectively engaging the grippable member, and a hydraulic mechanism on the housing body for delaying sliding of the housing body relative to the grippable member when an external force is applied to the housing body.
In another aspect, the invention relates to a connector latch which comprises a housing body having an internal cavity, a grippable member slidably coupled to the housing body, a sealed chamber defined within the internal cavity, the sealed chamber having a volume which varies with relative movement between the housing body and the grippable member, and a flow restriction for controllably releasing fluid from the sealed chamber to the internal cavity as the volume of the sealed chamber decreases.
In another aspect, the invention relates to a mechanical latch for connecting two mating parts of an electrical connector which comprises a housing body having an internal cavity, a grippable member slidably coupled to the housing body and attached to one of the mating parts, a gripping member adapted to selectively engage the grippable member and attached to the other mating part, and a hydraulic mechanism on the housing body for delaying movement of the housing body relative to the grippable member when an external force is applied to the housing body.
In another aspect, the invention relates to a method for connecting and selectively disconnecting a gripping member. The method comprises lowering a tool comprising a grippable member to the gripping member, bringing the grippable member in contact with the gripping member, pushing the gripping member against the grippable member until the gripping member engages the grippable member, and applying a tension to the tool for a duration determined by a hydraulic time-delay mechanism in the tool, and moving the tool relative to the gripping member to release the grippable member.
Other aspects and advantages of the invention will be apparent from the following description and the appended claims.